Image display projectors generate display images and project them onto display screens, typically for viewing by multiple persons or viewers. In a front-projection format, for example, the projector is positioned on the same side of the display screen as the viewers. The projector directs the display image to the display screen from which the image is reflected for viewing.
Some image display projectors, such as motion picture film projectors, typically are used in relatively large rooms, are positioned behind the viewers, and project the display image a relatively long distance to the display screen. Frequently, the display screen is positioned above the viewers and the projector directs the display images over the viewers at a relatively small downward elevation angle, which is defined as the angle of a central light ray from the projector relative to an axis that is normal or perpendicular to the center of the display screen. As a result of the relatively small elevation angle, the display image on the display screen appears generally rectangular and orthoscopic.
Other image display projectors, such as liquid crystal display (LCD) projectors, frequently rare used in relatively small rooms and are positioned relatively close to the display screen; either in front of or otherwise near the viewers. A LCD projector includes one or more transmissive liquid crystal displays as "light valves" that control light from an illumination system to form display images that are directed to the display screen by projection optics. Many color LCD projectors typically have one color LCD, such as an active matrix or passive matrix (e.g., super-twisted nematic (STN)) liquid crystal display, that controls red, green and blue spectral components of light to form a display image.
LCD projectors frequently are positioned on a table, cart, or other flat surface below the viewers' line-of-sight to the display screen. Alternatively, LCD projectors are sometimes mounted above the display screen (e.g., to a room ceiling) to direct the display image downwardly toward the display screen from above the viewers' line-of-sight.
A LCD projector directs the display image a relatively short distance to a display screen and, therefore, at relatively wide projection angles and at elevation angles of relatively large absolute value (e.g., greater than about 8 degrees typically, and as much as 25 degrees if it were physically possible). The absolute value of the elevation angles is referred to because LCD projectors mounted above and below a display screen would typically have negative and positive elevation angles, respectively. As a consequence of such a large elevation angle, the display image on the display screen appears trapezoidal and not orthoscopic. This is sometimes referred to as keystone distortion, which is generally unacceptable in many image display applications including motion pictures and computer displays.
Keystone distortion in LCD projectors typically is corrected by tilting certain optical components relative to the optical axis. One such system is described in U.S. Pat. No. 5,355,188 of Biles et al. fox Method and Apparatus for Distortion Correction in Optical Projectors. In this LCD projector, a Fresnel lens in the LCD projector is tilted to provide the projected display image with a differential magnification that offsets the keystone distortion. However, there is a limit to the degree that any lens may be tilted without causing image degradation due to the relatively poor imaging performance of lenses when used at steep off-axis angles. Another LCD projector in which keystone distortion is purportedly corrected by tilting certain optical components is described in U.S. Pat. No. 5,321,450 of Shapiro et al.
A limitation of such corrections for keystone distortion is that the corrections are suited primarily for a single elevation angle between the LCD projector and the display screen. However, LCD projectors frequently are used under different circumstances resulting in different elevation angles.
More specifically, it is frequently necessary to adjust the picture height or projector elevation angle to reach a fixed (e.g., permanent) display screen or to adjust the height of the image for best unobstructed and comfortable viewing. Nearly all LCD projectors have elevation-adjusting legs or feet for this purpose. A conventional LCD projector with an elevation angle greater than the keystone-corrected angle will form a display image with "positive" keystone distortion (i.e., the top of the image is wider than the bottom). A conventional LCD projector with an elevation angle less than the keystone-corrected angle will form a display image with "negative" keystone distortion (i.e., the top of the image is narrower than the bottom).